There is presently a significant industrial effort to develop and install new optical fiber transmission networks capable of extremely high data transmission capacity, and also to upgrade existing networks. In these networks, optical pulses represent binary bits of information. An obstacle to improving the capabilities of such systems is fiber chromatic dispersion which causes light having different frequencies to travel at different velocities in an optical fiber and results in optical pulses which increase their pulse width as they move within the fiber. There are additional effects, such as energy losses, other types of dispersion, and scattering interactions with the transparent glass medium which likewise degrade the received signal causing errors in the transmitted information. The problem of optical soliton transmission can be divided into two categories: (1) long-distance transmission based on dispersion-shifted fibers with periodic amplification, and (2) the improvement of existing networks based on standard monomode fibers.
One solution to the chromatic dispersion problem is the so-called Center Guiding Soliton Technique (CGST) described in "Guiding-Center Solitons In Optical Fibers," by Akira Hasegawa and Yuji Kodama, Optics Letters 15, 1443 (1990). With a proper choice of the initial amplitude and amplifier distance, the envelope of a nonlinear pulse (a guiding center soliton) propagates like a soliton over a distance much larger than the dispersion distance (the distance over which chromatic dispersion will approximately double the pulse width in the absence of other optical effects such as nonlinearity) provided it is periodically linearly amplified at distances much shorter than the dispersion distance. This technique is known to work well, but is limited by the condition that the dispersion length, Z.sub.d, must be much greater than the amplifier spacing, Z.sub.a. The dispersion length is proportional to the square of the pulse width. Consequently, for the short pulse widths necessary for ultrahigh data transmission rates, the dispersion length will be very short and CGST cannot be effectively applied. That is, for the improvement of existing networks based on standard monomode fibers, the dispersion length is of the same order as the amplifier spacing for the desired bit rates and, therefore, dispersion is quite significant, and the center-guiding soliton transmission is not applicable. It should be mentioned that the losses for most commonly employed optical fibers are approximately the same regardless of dispersion.
In "Nonlinear-Optical Loop Mirror," by N. J. Doran and David Wood, Optics Letters 13, 56 (1988), a nonlinear switching device for ultrafast processing based on the nonlinear propagation in a waveguide loop formed by connecting the output ports of a conventional optical coupler is proposed. The device operates on the nonlinear phase induced by self-phase modulation and does not require interferometric alignment, is robust, and is of simple construction. A single input to the optical coupler is split into two counterpropagating fields which return in coincidence to recombine at the coupler. The optical path is precisely the same for both propagating fields since they follow the same path but in opposite directions. When the coupling ratio is unity, the device simply acts as a mirror; that is, the input is reflected back upon itself when the power-coupling ratio of the coupler is unity. Otherwise (when the coupling ratio is not unity), the effect of propagation will no longer be identical since the phase modulation is intensity dependent. Only a small fraction of the available waveguide nonlinearity is used, however, since the splitting ratio must be close to 50:50 to achieve adequate modulation, and thus the differential phase delay between the two arms is low.
The output of the device proposed by Doran and Wood is nonlinearly dependent on input energy. Pulses having the correct input energy will be transmitted (switched) by the nonlinear optical loop mirror (NOLM). This applies for soliton input pulses as well as for constant beam input. Solitons are stable pulses created by a balance of the self-phase modulation and dispersion in the negative group velocity-dispersion regime in an idealized optical fiber. To leading order, in real fibers, an exact single soliton does not change shape in its propagation through the fiber rather it acquires a phase shift proportional to the distance. Even if the pulse is not an exact soliton, the effects of dispersion can be approximately balanced by the nonlinearity, and a pulse whose amplitude and shape are close to those of an exact soliton does not change significantly on propagation. When the NOLM is operated in the soliton regime, excellent switching characteristics can be obtained for bell-shaped pulses. For nonsoliton pulses, there is a requirement that the loop have sufficient length for dispersion to take effect, which in practice means a few soliton periods. Thus, the required loop length is reduced as the square of the pulse duration. For subpicosecond switching only a few meters of fiber would be required.
"Nonlinear Amplifying Loop Mirror," by M. E. Fermann et al., Optics Letters 15, 752 (1990), describes an improved exploitation of the nonlinearities of the NOLM by adding an amplifier positioned at one end of the loop in the situation where the coupler has a splitting ratio of exactly 50:50. For small optical input powers, the nonlinear amplifying loop mirror (NALM) operates in the linear regime, the mirror acting like an actual mirror and reflecting the pulses with no light emerging from the second port of the device. Since the amplifier is placed at one end of the loop at high optical input powers, the nonlinear refractive index leads to a change in the optical path lengths for light propagating in each direction, and to a phase shift therebetween. If the amplifier and the pulse lengths are short and the individual pulses do not saturate the amplifier, the pulse passing first through the amplifier experiences a G-times higher phase delay than does the pulse traveling in the other direction, where G is the gain of the amplifier. Thus, the NALM has been found to significantly improve the performance and versatility of the NOLM of Doran and Wood. In particular, the threshold power for switching is reduced and complete blockage of low-power radiation can be achieved due to the presence of a 50:50 coupler.
In "Adiabatic Amplification Of Solitons By Means Of Nonlinear Amplifying Loop Mirrors," by Masayuki Matsumoto et al., Optics Letters 19, 1019 (1994), the transmission properties of a NALM that act as an adiabatic amplifier for solitons is investigated. The authors discuss the use of NALMs in long-distance soliton transmission to suppress the accumulation of linear waves as an extension to "Suppression Of Noise Accumulation In Bandwidth-Limited Soliton Transmission By Means Of Nonlinear Loop Mirrors," by Masayuki Matsumoto et al., Optics Letters 19, 183 (1994), wherein it is numerically shown that the amplification and accumulation of noise and other dispersive waves in bandwidth-limited soliton transmission can be suppressed by using NALMs that selectively amplify solitons. The periodic insertion of NALMs in a transmission line permits soliton transmission to occur so long as the NALM acts as an adiabatic soliton amplifier and does not generate appreciable linear dispersive waves. In this paper, adiabatic amplification is applied for soliton transmission where the pulse width of the soliton is short enough such that the soliton deformation is also adiabatic (the loss distance being larger than the dispersive distance). Only the pure adiabatic case is discussed by Matsumoto et al. In other words, parameters are selected such that the initially propagated soliton essentially remains a soliton during the amplification and decay processes. A near-adiabatic soliton amplification with an amplification ratio of approximately 2 results. Thus, by a combination of adiabatic soliton amplifiers made by using NALMs with some gain-control mechanisms, filters, or saturating optical elements, long-distance transmission of solitons having picosecond durations is possible.
Accordingly, it is an object of the present invention to use nonlinear amplifying loop mirrors to recover soliton pulses nonadiabatically deformed by losses.
Another object of the invention is to use nonlinear amplifying loop mirrors at varying distances to recover soliton pulses nonadiabatically deformed by losses.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.